Hybrid electric vehicles (HEV's) include an internal combustion engine and an electric traction motor to provide power to propel the vehicle. One method of increasing fuel economy in an HEV is to shut down the engine when the overall power demands are low. However, if the overall power demands increase such that the traction motor cannot provide enough power to meet the demand, or if the fraction battery state of charge (SOC) is below a certain threshold, the engine must be activated to supplement the power output of the traction motor.